1. Field of the Invention
The present invention relates to a scramble apparatus and a descramble apparatus which can be used by persons of limited skill in scrambling and decoding operations which are required to transmit or preserve digital-coded signals, and more particularly, to a scramble apparatus which effectively controls the degree of scramble according to purpose and a descramble apparatus for reproducing a scrambled signal and to a scramble transmission apparatus comprising the scramble apparatus and the descramble apparatus.
2. Description of the Prior Arts
Conventional effective scramble control is used in TV broadcasting. Regarding a scramble apparatus for handling a digital signal, sound signal scramble is described in Television Institution Technical Report Vol. 15, No. 14, pp.1-6.
The conventional scramble apparatus comprises an exclusive OR calculation device for applying random numbers to an inputted signal; a random number generator for generating a series of random numbers by using a scramble key; and a control circuit, disposed between the random number generator and the exclusive OR calculation device, for controlling the random number to be applied to an inputted signal.
In the construction of the conventional scramble apparatus, the exclusive OR calculation device performs an logical exclusive OR of the inputted signal by using a series of pseudo random numbers generated by the random number generator. In this manner, the inputted signal is bit-inverted. The control circuit controls the bit inversion rate of inputted signals by appropriately switching the application of the random numbers to the inputted signals. In performing decoding, the exclusive OR calculation device performs an logical exclusive OR of a scrambled signal by using the random numbers generated by the random number generator. The pseudo random number generated by the random number generator is determined by the scramble key and decoding can be accomplished by only persons having the scramble key.
In the above-described conventional scramble apparatus and an apparatus for reproducing the scrambled signal, when the inputted signal has been fixed length-coded, the position of each code and the code length thereof are known if a bit of data is inverted at random. Therefore, a code to which random numbers have not been applied is correctly reproduced and the degree in which a person not having the scramble key can reproduce the scrambled signal can be controlled by a random number-applying rate.
The above-described conventional scramble apparatus has the following problem: When an inputted signal includes a variable length code, even a variable length-coded portion is converted into a random bit pattern by scramble processing. This is because the signal is bit-inverted at random. In the variable length-coded code, the length of the code is not constant and all bit patterns which can be expressed by each code length do not exist in a code book. When several bits of the code are inverted, the code becomes an unproducible code because the code is not provided in the code book or is interpreted as a code having a different code length from the original code. As a result, the start position of the following code is erroneously found and subsequent data cannot be reproduced.
Another disadvantage of the conventional scramble apparatus is that in the case of compression-coded data, the data is often coded in several parameters and the degree of influence is different depending on a the particular parameter used when the data is extension-processed. Thus, when the data is scrambled at random, control cannot be favorably accomplished.